I'd be interested to know what you believe the ups and downs are, aside from speed.
Another one is the ORM, Ecto. It sticks much closer to SQL and I strongly prefer it over ActiveRecord. Ecto also includes the concept of changesets, which is a wonderful way to deal with validating user data and definitely an improvement over the Rails approach.
And finally, while I wouldn't necessary call it a 'failure' on Rails' part, the channels functionality in Phoenix is awesome for the more real-time apps that we have these days, and Phoenix is generally quite a bit faster in general than Rails.
Beyond that there's barely any resemblance.
I really do like the coding convention though that lack of parenthesis can indicate a function meant to be a part of a DSL.
...Gods, what a relief. Especially the automatic formatting on save is just a lifesaver. At certain point I would not mind if there is one formal syntax enforced like Go does.
Ecto is practically the only library I use where I don't pay attention to parentheses. And the formatter has me covered afterwards.